Garden Myths - Learn the truth about gardening

Bloom Booster – Fertilizer Nonsense #5

Bloom Boosters are said to increase the number of flowers on your plant. Nonsense. They don’t work, and can actually make your soil toxic, making it more difficult for your plants to grow.

Bloom booster fertilizer - who needs it

Bloom booster fertilizer – who needs it? Clematis recta growing just fine at Aspen Grove Gardens without any fertilizer. Photo by Robert Pavlis

Bloom Booster – What is it?

Bloom Booster fertilizer is a fertilizer with a high middle fertilizer number – it is high in phosphorus. It may also have a higher than normal amount of potassium.

There is no such thing as a Bloom Booster fertilizer!

Any fertilizer with a high level of phosphorus can be considered to be a bloom booster fertilizer, even if the label just calls it fertilizer.

If you look at the fertilizer numbers in bloom booster products you quickly realize that every manufacturer’s formulation is different. That’s odd! If there was a special formulation that boosts flowers you would think all manufacturers would use the same formula? They don’t, because there is no such thing. These are just packages of fertilizer with the words ‘bloom booster’ slapped on the label.

Bloom booster fertilizer is nothing more than a marketing gimmick to get you to buy more fertilizer. Many people grow plants for the flowers, and so manufacturers are using your desire for more flowers to sell more products.

Bloom Boosters, Do They Work?

Will a high level of phosphorus increase the number of flowers?

The answer to this question is a bit more complex than it seems. Plants need a certain amount of phosphorus to grow properly. If they get enough phosphorus along with the other needed nutrients, plants will grow to their best ability and produce lots of flowers. If they can’t get enough phosphorus and other nutrients then they under perform.

Adding phosphorus to soil that already has enough phosphorus, or adding it to one that is deficient in one of the other nutrients, will NOT make plants grow better. You are just wasting a natural resource or worse, making your soil toxic.

Adding a bloom booster to soil that already has lots of phosphorus, will NOT increase the number of blooms.

It turns out that in North America, few garden soils have a deficiency of phosphorus, which means that in most of these soils bloom booster does nothing to boost blooms.

If you check bloom booster fertilizers you will notice that some include minor nutrients, along with some nitrogen and potassium. If the fertilizer fixes any deficiency in these other nutrients, plants will perform better. Except for nitrogen, most garden soils do not have a deficiency in these other nutrients.

Do Plants Use a Lot of Phosphorus?

Lets have a closer look at the nutrients plants need. How much phosphorus do plants need?

When rose tissue is analyzed, the fertilizer numbers are in the ratio of 6-1-4 (ref 1). They contain much less phosphorus than nitrogen or potassium. According to Jeff Gillman (ref 3), “there is usually more calcium, magnesium and sulfur in plant tissue than phosphorus”.

So why would a fertilizer ratio of 1-5-1 (Peters Professional Super Bloom Booster), be good for plants?

Phosphorus is critical for plant growth, but plants don’t need a lot of it. The American Rose Society says “Commercial  growers of roses for cut-flower production typically use fertilizers with a 3-1-2 NPK ratio.” I think these guys know how to produce blooms!

Plants do not need high levels of phosphorus to bloom well.

Phosphorus Can be Toxic

High levels of phosphorus in the soil ties up iron so that plants can’t absorb it. This leads to an iron deficiency in the plant, leading to interveinal chlorosis.

Excess phosphorus also inhibits the development of mycorrihizal fungi which are very important to plants. These fungi provide water and phosphate to the plant. For more details on this see Mycorrhizae Fungi Inoculant Products

Excess phosphorus is toxic to plants.

Does Potassium Boost Blooms?

Some of the bloom boosters also provide an increased level of potassium, and some people feel that potassium is important for flower production.

Potassium is essential for plants to grow and flower properly, but high potassium levels will not increase flowering. As reference 2 points out – flowering is controlled by hormone levels – not nutrient levels.

What About Containers?

In reference 3, Jeff Gillman quotes Timothy Broschat and Kimberly Klock-Moore from the University of Florida “Most container grown plants require only minimal amounts of P for optimal growth and … applications of high P fertilizer will not promote either roots or shoot growth in plants as popularly believed.”

Don’t use bloom boosters for containers.

Phosphorus Does NOT Promote Flowering

There is little evidence that phosphorus promotes flowering. Bloom boosters are not needed, nor do they do help your plants, unless you have a phosphorus or potassium deficiency in the soil. The only way to know this, is with a soil test.

Stop adding phosphorus to your soil!


1) The American Rose Society – Phosphorus Fallacies – too Much of A Good Thing:

2) The Story of Flowers – The Potassium Myth:

3) “The Truth About Garden Remedies”, book by Jeff Gillman

If you like this post, please share .......
Robert Pavlis
Editor of
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

I hope you find Garden Myths an educational site that helps you understand your garden better.

19 Responses to 'Bloom Booster – Fertilizer Nonsense #5'

  1. Theryl McCoy says:

    Fertilizer nonsense! I love it.
    So a soil test is essential. I get that When doing a soil test, do plants require different NPK ratios of nutrients in the soil? For instance how much is too much Phosphorus in soil? When you say North America soils have ‘pleny’ of phosphorus… how much is ‘plenty’? Would that depend on the plant’s requirements, or flowering stage vs vegetative stage?

    • That is a good question – how much P is enough P. it turns out that most soil testing labs in North America base their “adequate P level” on agricultural levels – not on levels required for gardens. As a result most testing labs recommend too much P to be added to soil – for the home owner. I am still not sure what level of P is good for garden soils but i am working on getting some good numbers.

  2. Michael says:

    I have been reading your blogs and have gained many insights to gardening, thank you for that. Some of us are now heading off into hydro type systems. We are starting with substrates that are bare (sponge, rockwool, coco noir, clay pebbles, etc.,) of any nutrients and condition our own water to feed the plants during their life cycles. This means different sources of food / minerals during seeding, vegetative states and finally the flower cycle. My question/s may be a little off but I wonder what are your thoughts about mycorrizhal innoculant in these substrates, and what fertilizers / nutrients are best at each stage (while reading about bloom boosters)? Besides my question, will bloom boosters make a difference here or is the standard feeding for flower stage enough to negate any benefit from bloom booster type products?

    • If I understand your question correctly, these are going to be hydroponic systems. As far as I know mycorrhizal fungi do not grow in water, in which case they would add no value. If they do grow in water, do you need them. The main benefit they provide is to collect phosphorus and water from the soil. Water is no issue in hydroponics, and the added fertilizer easily adds enough P.

      Plants absorb the nutrients they need. Provided that the hydroponic solution has enough phosphorus, adding bloom boosters adds no value.

      • Michael says:

        Thank you, very informative. The system is not pure hydro, it uses a substrate usually comprised of rockwool and coco noir. Will mycorrhizal fungi work in this media or the fact that all nutes are mechanically introduced in the H2O make them useless?

        • I don’t think the substrate matters – it is there to be inert. I think that there may be some benefit in very low nutrient levels, but if P is normal or high it will inhibit fungi from growing. This is the same problem as potted plants. If fertilizer is added as required, mycorrhizal fungi serve no purpose.

      • I did some digging and mycorrhizal fungi do populate aquatic plants, so it is reasonable they can also populate plants in a hydroponic system.

        That does not negate the comment I made. Do they add any value?

  3. Basil says:

    Hi Robert, do you have any comment on 20-20-20 fertilizer? Would you recommend that for a healthy vegetables or flowering plants?

    • Absolutely not!

      Fertilizer needs to be added to your soil to replace the nutrients that are lacking in the soil. Without a soil test you do not know what is lacking.

      Your soil might have lots of P – why add more? Too much is toxic.

      The nutrient that may be lacking in most soils is nitrogen. A 5-0-0 makes sense. Except for urea you can’t get just nitrogen, so try a 5-0-1, or something with a small last number and large first number.

      See my post called All Tomatoes need The Same Fertilizer.

      Also – use the menu called “List of Topics” and read the ones about fertilizer.

  4. Lisa says:

    I have a morning glory which produces only a few massive blooms at the end of the season ( loads of leaves ) and it grows like crazy. It grows from a 6ft by 2ft brick container, gets sun ( SW France ) and has started to climb everywhere. My questions are : How do I know if the soil lacks phosphorous, and if it doesn’t need it what do you suggest for more flowers? Thanks a lot..

    • The only way to know if you have a phosphorus deficiency is to do a soil test.

      For the morning glory, I would suggest joining a FaceBook gardening group for France. Ask your question there, and include the name of the morning glory. Try to find someone who has knowledge of your native soil, and growing conditions, and who has grown the particular plant you have. There are lots of types of morning glories.

  5. tolga erok says:

    Rob, can you please do a blog on rock dust and sea-90, is it myth or fact that these products work? as it seems to be the new trend in adding to soils these days….

    • Rock dust is on the long list of topics to be done – never considered Sea-90.

      Rock dust. Rock dust is basically rock – small bits of rock, but still rock. Rock takes hundreds of years to degrade. In the short term it does very little for your soil.

      Sea-90. Sea salt is mostly sodium chloride which is toxic to soil and plants if the concentration gets high enough. The minor minerals in it are already present in most soils. To be blunt – this is a dumb product to add to your garden soil.

  6. anijhuis says:

    Hey would like permission to reblog some of your posts – referring to your blog of course

  7. rogerbrook says:

    Over here in England the miracle material that is rich in phosphate is bonemeal. It is completely useless – for anything!
    I agree adding phosphate fertiliser to the soil in most circumstances is a waste of time because the soil has its own high and sometimes harmfully excessive reserves.
    It is gardening lore over here that potash encourages flowers! I have blogged about this myth.

    As you say if a plant has good all round nutrition, often without using any fertiliser at all it will grow and flower well

    • Bonemeal is also very popular in North America. It cures everything in the garden !!

      I also believe that bonemeal does not degrade very fast – but have not found a good reference about this, to verify it. I think it just sits in the ground like a bone.

  8. Tony Cuthbert says:

    Hi Robert,
    I really like the way your blog makes me question what are sometimes orthodoxies in horticulture.
    I would wonder if there is this simple correlation between the ratio of major nutrients in a plant with the ratio of major nutrients in a fertiliser. The rate of uptake of each of the elements NPK may be different; their storage and transport around the plant may be different; their use within the plant may be different. You are measuring the total amount of NPK in the plant whether they are part of useful molecules, waste molecules, being stored or being transported. The amounts of these nutrients in a plant are tiny compared with the amount of carbon and water and their ratio and amounts within the plants changes over the seasons. Each specie and variety of plant will have different needs for each of the major elements. So to give the impression that application of inorganic fertilisers is simple and throwing it on willy nilly does not mean that your plants will prosper. Furthermore, the changing of the salt content of the soil water can lead to reverse osmosis and water loss from the plant roots – not something that will encourage root growth and healthy plants.
    There may only be tiny amounts of NPK in home made compost but plants only need tiny amounts. I only use organic NPK or rock dust to add these elements to my soil.

    • I agree with everything you say. The whole idea of matching fertilizer to plant type makes no sense. Adding huge amounts of commercial fertilizer is rarely needed in the landscape garden. Nitrogen is the only nutrient that is usually deficient in garden soils. Plants use quite a bit of it, it is easily converted to other forms in the soil or air, and it easily moves through soil – basically washing away. A bit of compost is all most plants need.